The invention is concerned with photovoltaic devices, particularly with improving the quality of molybdenum contacts used in such devices.
The use of photovoltaic cells for the generation of electrical energy is well known. The cells contain semiconductor materials exhibiting the photovoltaic effect and these are typically realised in solar panels comprising a number of cells. Solar energy represents a clean, environmentally friendly source of electricity and, although at present it accounts for a small fraction of global energy consumption, it is also a rapidly expanding industry. Of the thin film photovoltaic technologies in mass production at this time, CuIn1-xGaxSe2-ySy (CIGS) and CuInS2 (CIS) have demonstrated the highest efficiencies. It is generally accepted that as device efficiencies increase to high levels, incremental improvements become of greater importance.
Molybdenum is used for the back contact in CIGS/CIS cells due to its excellent adhesion and low contact resistance with the materials concerned. Molybdenum is also inert and stable during the aggressive CIGS deposition process.
Additionally an intermediate MoSe2 layer is formed during CIGS deposition, which provides an ohmic contact between the absorber and the back contact. Therefore the main focus when depositing Mo has been on providing a material with low sheet resistance and good adhesion. In order to achieve both of these properties it has been shown by Scofield et al, that deposition of a Mo bi-layer is necessary (Scofield et al, Thin Solid Films, 260, (1995), 26.). This involves deposition of a high pressure Mo layer to give a well adhered film under tensile stress, followed by deposition of a Mo layer at lower pressure to give low sheet resistance (this film is under compressive stress).
Attainment of CIGS material of the desired texture for highly efficient devices is heavily dependent on the structural properties of the Mo layer (Contreras et al, Thin Solid Films, 361-362, (2000), 167.). Properties such as morphology, grain size, and stress state are variables that are likely to affect the CIGS nucleation and growth process. Work by NREL demonstrated that it was desirable for the Mo to be crystalline and in the (110) orientation, the so-called “fish-like” morphology (Dhere, NREL report, FSEC CR1416-03, (2003)). Work by Assmann et at confirms this (Assmann et al, Applied Surface Science, 246, (2005), 159.).
As CIGS devices are grown in the substrate configuration, the Mo effectively serves as a growth layer for the rest of the device and so a high quality Mo layer is essential to achieve high efficiency devices. Improvement of the (110) orientation of the Mo and the ability to control its crystalline structure without adversely affecting other properties would represent significant advances in the art. This ability allows for tuning of the Mo layer properties to device requirements, which is important as small variations in the crystalline structure of CIGS can strongly affect the series resistance and the fill factor of devices (Siebentritt et al, Prog Photovolt Res & Apps, 18, (2010), 390.). Furthermore it is likely that for different CIGS deposition processes, the optimum Mo texture is likely to be different and so being able to tailor morphology for an individual process is important.
One final function that Mo can perform is in the control of sodium diffusion into the CIGS material. In order to obtain the highest efficiency devices, the presence of sodium is necessary when the CIGS material is deposited (Hedström et al., Proc. of the 23rd IEEE Photovoltaic Specialists Conference, (1993), 364., Niki et al., Prog. Photovolt: Res. Appl., 18, (2010), 453.). In laboratory cells this sodium is generally supplied by diffusion from a soda lime glass substrate, through the Mo layer and into the CIGS material. Therefore the microstructure of the Mo can be tailored to control this sodium diffusion. However for an industrial process, close control of sodium content is essential to achieve consistent high performance and efficiency and so relying on diffusion from the glass would be considered difficult. Sodium content and mobility in glass can vary depending on glass origin, composition, age and storage conditions. To prevent uncontrolled sodium diffusion from the glass, a sodium barrier layer is used between the glass and the molybdenum coating. Sodium can then be introduced in a controlled manner via deposition of a sodium compound, such as sodium fluoride (N. G. Dhere, Solar Energy Materials and Solar Cells, 95, (2011), 277.). Examples of common sodium barrier layers would be SiO2 or SiN. When using barrier layers to restrict sodium diffusion it is desirable that Mo is grown with a densely packed grain structure to give greater protection from uncontrolled Na diffusion and with as few pinholes as possible, i.e. a high quality film is needed.
Control of Mo growth can be achieved by varying the deposition parameters; however this invariably changes other properties of the material. A means of enhancing the orientation of the material without affecting other properties would therefore be desirable. As mentioned previously, for high efficiency CIGS devices the Mo should be in the (110) orientation. By the invention, this (110) orientation is enhanced and can be tuned as desired.
Barreau and Bommersback (Proceeding of EUPVSEC, Valencia, (2010), 3BV.2.23) discuss the improvement of Mo orientation by using ZnO, however they use rather thick layers >100 nm to improve growth. According to this disclosure, increasing the ZnO thickness from 0 to 50 nm, gives an increase in the intensity of the (110) signal by a factor of ˜1.5. A further increase to 150 nm ZnO leads to a 3-fold increase after which the effect levels off. Overall the ZnO layer gives a 4.5-fold increase in (110) intensity. No data is presented concerning ZnO layers having a thickness between 0 and 50 nm. The implication is that the data obtained for other thicknesses may be extrapolated over this range.